The present invention relates to an apparatus for the vaporization of a heavy hydrocarbon feedstock, by mixing the hydrocarbon feedstock with superheated steam.
A well-known process for converting hydrocarbon feedstock to obtain valuable gaseous (mainly olefins) and liquid products is the so-called thermal cracking process. The hydrocarbon feedstock is normally diluted with superheated steam in order to favor the vaporization of the hydrocarbon feedstock, prior to introducing the vaporized hydrocarbon feedstock into the cracking section of a furnace. Such a process is also called steam-cracking and reduces the hydrocarbon partial pressure and contact time during the cracking phase. If processing heavy hydrocarbon feedstocks, e.g. materials with a boiling range above 300.degree. C., the vaporization of the liquid material is normally carried out in a plurality of stages. The hydrocarbon feedstock is first preheated, whereafter the still liquid feedstock is admixed with superheated steam to form a gas/liquid two phase mixture while simultaneously heating the liquid. Further heating the mixture of the steam and liquid droplets to partially vaporize the liquid, fully vaporizing the liquid by introducing a further amount of superheated steam into the flow of steam and partially vaporized liquid. The steam is, for example, added to the hydrocarbon feedstock by means of a nozzle wherein steam is introduced as an annulus around a core of the hydrocarbon feedstock.
It has been found that in the known nozzles coke forms in the flowline, especially at the location where steam is introduced for the final vaporization step. Although the occurrence of coke formation cannot be fully scientifically explained, it might be assumed that the following happens in a coke formation. In the partially vaporized liquid, the lighter fractions are in the vapor phase whereas the heavier fractions are still in the liquid phase. When the partially vaporized liquid is further contacted with superheated steam to finalize the vaporization, the temperature of the liquid droplets upon contact with the steam will immediately rise to the steam temperature. Part of the lighter components in the liquid drops will immediately be vaporized, having as consequence that a skin of heavy material, especially for the larger drops, can be formed via evaporation and chemical change and further evaporation will be limited by the slow diffusion of lighter material through the outer skin of the droplets. The inability of the droplets to completely evaporate would lead to coke formation upon further subjection to the high steam temperature. Furthermore, drops of liquid reaching the wall of the mixing nozzle would easily stick onto the wall thereby experiencing a long residence time, during which the liquid droplets are subjected to high temperatures. The long residence time of the liquid droplets on the wall of the mixing nozzle will result in a high risk of coke formation on said wall. Due to coke formation the passage for liquid and steam in the mixing nozzle is diminished, resulting in an increase of the pressure drop over the mixing nozzle.
When the pressure drop increase becomes excessive the thermal cracking operation must be stopped for cleaning the mixing nozzle. It will be understood that this is a time consuming and costly operation.